This invention relates generally to the field of nanofabrication. More specifically, the invention relates to methods of fabricating and modifying nanostructures by patterning the chemical functionality of molecules that overlay a substrate. The invention provides for chemical and spatial complexity and precision such that even nanostructures are scaled down using molecular science. The invention is suited for use in fabricating devices such as sensors, in directional growth or placement of cells, and in self assembly of components into devices.
The ability to construct increasingly small and complex structures is of great importance in the fabrication of advanced electronic, optical, photonic, and sensing devices, as well as in other applications. There continues to be increased interest in creating smaller and more precise patterns. Of particular interest is creating patterns on the mesoscale or even the nanoscale. Some of the methods and attempts relating to creating smaller patterns relate to molecular science.
Conventionally, electron, photon, or ion exposure has been used to induce massive changes and bond breakages in polymer resists located on surfaces. Either the exposed or unexposed portions of polymer resists are then removed to leave a pattern. These and other lithographic approaches have been used to pattern surfaces. Lithography techniques involve printing on a surface in either an additive or subtractive process. In an additive process, the pattern that is printed onto a surface creates a new structure or modification to a structure. In a subtractive process, the pattern serves an intermediary role, protecting a portion of the surface while other portions are removed. Such common methods include electron beam lithography. See P. Rai-Choudhury, Ed. SPIE handbook of Microlithography, Micromachining and Microfabrication (SPIE, 1997) vol. 1. Other techniques utilize a scanning probe microscope (SPM). See H. Sugimura, N. Nakagiri, J. Am. Chem. Soc. 119, 9226 (1997); M. A. Reed, J. Chen, C. L. Asplund, A. M. Cassell, M. L. Myrick, A. M. Rawlett, J. M. Tour, P. G. Van Patten, Appl. Phys. Lett. 75, 624 (1999); S. Hong, J. Zhu, C. A. Mirkin, Science 286, 523 (1999).
Despite all these different possible methods of creating nanostructures, significant problems and limitations remain. One problem has been that the chemistry used has been imprecise. Typically, polymer molecules are destroyed by irradiation with electrons, ion, and/or photons. These induced reactions are not well-defined and are imprecise. There have been few exceptions to the general problems of poorly defined resists and poorly defined products of induced reactions. One attempt to address one aspect of these problems has been by using monolayers as resists to replace more typical polymer resists in which molecules are randomly oriented, overlay one another in random orientations, and the thickness of the film varies. See R. C. Tiberio, H. G. Craighead, M. Lercel, T. Lau, C. W. Sheen, D. L. Allara, Appl. Phys. Lett. 62, 476-478 (1993). In this case, the reaction products and resulting structures remain ill-defined.
One attempt at patterning surfaces is based on selective attachment of monolayers and multi-layers. See A. Hatzor and P. S. Weiss, Science 291, 1019 (2001). This method is useful in creating further patterns.
Non-lithographic methods of patterning have been used to pattern surfaces. These methods have been referred to as soft lithography. See R. S. Kane, S. Takayama, E. Ostuni, D. E. Ingber, G. M. Whitesides, Biomaterials 20, 2363 (1999). These techniques include microcontact printing, patterning using microfludic channels, and laminar flow patterning. These patterned surfaces have been further used to pattern proteins and cells.
Microcontact printing relies upon the molecular self assembly of self-assembled monolayers (SAMs) to provide for surface patterning. In microcontact printing, a molecular stamp is inked and then applied to a surface. The molecular stamp is then removed, leaving the ink on the surface. Typically, the surface is then immersed in a solution that promotes formation of the self-assembled monolayers on that portion of the surface that was not inked thereby creating a pattern.
Microcontact printing has been combined with chemical reaction. See L. Yan, C. Marzolin, A. Terfort, G. M. Whitesides, Langmuir 13, 6704-6712 (1997); L. Yan, X. M. Zhao, G. M. Whitesides, J. Am. Chem. Soc. 120, 6179-6180 (1998). A reactive SAM is placed on a substrate, and this reactive substrate is stamped. The mixed SAM can then be reacted further. One advantage of reactive SAM microcontact printing is that more types of functional groups are available than would be using other chemical methods.
Yet problems remain. The precision available using microcontact printing techniques is limited. Further, a number of steps may be required to create complex patterning.
Another attempt at patterning surfaces involves dip-pen lithography. Dip-pen lithography combines atomic force microscopy (AFM) and SAMs technology to provide for direct writing of a pattern on a surface. SAMs are transferred down the tip of the pen to the substrate. Dip-pen lithography provides a direct method of patterning, but is problematic in at least several respects. For example, the process is a very slow serial process. Although it is possible to use tips in parallel, only limited increases in speed are achievable and the resolution is limited.
Generally, in soft lithography, resolution and accessible patterns, and chemical functionality are all limited. One partial exception has been nanoscale patterning of hydrogen terminated silicon surfaces. See J. W. Lyding, T. C. Shen, J. S. Hubacek, J. R. Tucker, G. C. Abeln, Appl. Phys. Lett. 64, 2962 (1994). Patterning occurs when electrons field emitted from the probe of a scanning tunneling microscope locally desorb hydrogen, converting the surface into clean silicon. The limitation of this method is that the hydrogen monolayer may not be chemically modified to tailor surface properties further; silicon can be, but has very limited accessible chemistry.
As can be seen from the foregoing discussion, creating devices with nanoscale structures or chemical patterns remains a considerable problem. Thus, a need exists in the art for a method of creating spatially and chemically precise nanostructures or chemically patterned materials or substrates. Such materials would be useful in electronic component fabrication, sensor construction, component assembly, and other applications.
It is therefore an object of the present invention to provide a method for creating such structures that greatly improves the state of the art.
It is another object of the present invention to provide a method of creating nanoscale structures that results in structures that are chemically precise, spatially precise, and simultaneously both chemically and spatially precise.
It is a further object of the present invention to provide a method for creating nanoscale structures that can be chemically modified.
It is a further object of the present invention to provide a method for creating stable nanostructures that can be chemically modified.
It is a further object of the present invention to provide a method for making nanoscale structures that permits complex patterns to be made.
It is a further object of the present invention to provide a method for making nanoscale structures that permit a variety of structures to be created or modified.
It is a further object of the present invention to provide a method of creating nanostructures that allows a number of different nanostructures to be constructed in a single fabrication process or in a series of simple steps.
Yet another object of the present invention is to provide a method of creating patterns through chemical functionalization.
A further object of the present invention is to provide a method of creating patterns that is chemically flexible.
A still further object of the present invention is to provide a method that can use substrates prepared with lithography, soft lithography, or other techniques.
Another object of the present invention is to provide a method that provides for the creation of devices that can be used as or in the process of creating chemical sensors.
Yet another object of the present invention is to provide a method that provides for the creation of devices that can be used as or in the process of creating biological sensors.
Other objects of the invention will become apparent from the description of the invention and that which follows.
The invention involves novel methods of patterning a substrate surface. The method provides for covering the surface with a first plurality of molecules. The molecules can be a bound monolayer, a bound film of greater thickness, a partial monolayer, or a film that only partially covers the substrate. This partial coverage may be selected by the specific chemical interaction between the plurality of molecules and the materials that comprise the substrate, and may be induced by selective attachment.
The invention provides for selecting particular internal bonds within the plurality of molecules. Then, the selected internal bonds are broken or reacted to form one or more new functional groups. This reaction can be accomplished using electrons, photons, ions, excited atoms or molecules, heat, friction, mechanical contact, or electrochemistry. These can be patterned by scanning, through the use of masks, mechanical contact, and/or projection. Because the invention provides for forming new functional groups selectively throughout the substrate surface or in designated patterns, these newly created functional groups provide advantages.
In particular, a reactant can be introduced to the surface of the substrate such that a chemical reaction will occur between the reactant and the newly formed terminal functional group, but there will be no reaction with the portion of the substrate surface where new functional groups were not selectively created. Alternatively, reactants can be introduced that react with functional groups originally present on the substrate surface but do not react with the newly created functional groups. Further, reactants can be introduced that react both with the originally present functional groups and the newly created functional groups to produce different products. These nanostructures can be used for the selected deposition of nanostructures or components; for the selective growth, shaping, or attachment of cells; as the base for the creation of a sensor or sensors; or for other purposes related to nanotechnology.
The patterning of the present invention therefore provides significant advantages in creating or modifying nanostructures. Nanostructures can be created or modified at the molecular level. This can result in smaller nanostructures through a chemical process. These nanostructures can have controlled chemical functionality, and thus controlled chemical, physical, and other properties.